Contemporary systems typically comprise a solenoid-operated purge valve that is under the control of a purge control signal generated by a microprocessor-based engine management system. A typical purge control signal is a duty-cycle modulated pulse waveform having a relatively low frequency, such as 10 Hz. The modulation ranges from 0% to 100%. The response of certain conventional solenoid-operated purge valves is sufficiently fast that the valve follows to some degree the pulsing waveform that is being applied to it, and this causes the purge flow to experience similar pulsations. Such pulsations may at times be detrimental to tailpipe emission control objectives since such pulsing vapor flow to the intake manifold may create objectionable hydrocarbon spikes in the engine exhaust. Changes in intake manifold vacuum that occur during normal operation of a vehicle may also act directly on the valve in a way that upsets the control strategy unless provisions are made to take their influence into account, such as by including a vacuum regulator valve. Moreover, low frequency pulsation may produce audible noise that may be deemed disturbing.
It is known that maintenance of sonic flow will make a purge system quite insensitive to typical fluctuations in engine intake manifold vacuum that occur during engine operation. However, insofar as the applicants are aware, the possibility of maintaining sonic flow by configuring the canister purge valve's head and seat to cause sonic flow when the valve is open and the positive pressure across it is in excess of a certain minimum has not heretofore been recognized. Accordingly, a general aspect of the invention is to provide a canister purge system incorporating such a valve. More specific aspects relate to various constructional features, and some of these features may be useful in canister purge systems and valves that do not necessarily employ the novel valve head and seat for achieving and maintaining sonic flow in a canister purge system and valve.
The present invention also relates to an improved purge valve for an evaporative emission control system characterized by improved linearity and insensitivity to variations in intake manifold vacuum communicated to the valve so that better control of canister purging can be achieved. In addition to improvements that reside in constructional features of the purge valve, improvements reside in how it is operated by the purge control signal, particularly when the valve is to operate in the direction of closing. An important advantage of the inventive valve is that an additional vacuum regulator valve becomes unnecessary in view of the sonic flow characteristic of the inventive valve.
The foregoing, along with additional features, and other advantages and benefits of the invention will be seen in the ensuing description and claims which are accompanied by drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.